Petrophysics deals with the exploration and development of oil and gas reservoirs. It also involves the evaluation of potential reservoirs for water production and the storage of carbon dioxide and also for the generation of geothermal power. A general workflow for characterizing and modeling the reservoirs first involves defining regions, which are likely to have hydrocarbon reservoirs. Seismic surveys are used to define sub-surface structures, which may have reservoirs. Wells are drilled through those structures. Measurements from the drilled wells are then used to assess the rock formation in the reservoir to determine hydrocarbon content. Reservoir models are then built based on the geological, geophysical, and petrophysical information of the reservoir. Reservoir engineers use these models to plan oil production from the reservoir. Once a reservoir has been identified and oil production from the well has been determined to be economically viable, additional wells are drilled in the reservoir and the reservoir model is further refined as more information becomes available.